Application of time-frequency filtering to chirp coded tissue harmonic imaging

In this simulation study, the use of coded excitation methods for improving the performance of harmonic ultrasound imaging systems is described. Specifically, the use of chirp coded transmission waveforms is examined with the objective of extracting the second harmonic backscattered signal within one transmission cycle. An efficient method for predicting the transient nonlinear field from a focused transducer has been developed. Filtering schemes which utilize properties of chirps in the joint time-frequency domain were examined. Such filters take advantage of the fact that the harmonic components of a backscattered chirp are separable in time-frequency space. Specifically, filters based on the fractional Fourier transform (FrFT) and Chirplet transforms were investigated. The effectiveness of both schemes in eliminating the undesired compression sidelobes in the case of non-overlapping echoes has been demonstrated through simulations.